Spectrometers, such as hyperspectral imaging devices, are useful in a large number of applications. For example, hyperspectral data can be used in connection with target recognition for hard targets. As another example, hyperspectral data can be used to retrieve atmospheric properties for atmospheric detection systems.
In order to sample a spectrum in hundreds of colors, a hyperspectral instrument requires a large spectral field. In addition, in order for the data to be most useful, precise co-registration between colors is required. As a result, conventional hyperspectral dispersive spectrometers having a single optical path and a single focal plane array (FPA) have required a large optical field with low distortion. In order to accommodate the large field, a traditional common path hyperspectral dispersive spectrometer requires an FPA that is very large in one dimension. Because of the large optical field and the large FPA, traditional common path hyperspectral spectrometers can be difficult to manufacture.
As can be appreciated by one of skill in the art, dispersion from a diffraction grating will contain multiple wavelength orders. As a result, some traditional spectrometers separate the signal from different wavelength orders by using multiple spectrometer legs and multiple dispersive elements. Hyperspectral systems that split the spectrum into multiple optical paths with multiple FPAs have been developed. Although such systems avoid the need for a single, large FPA, co-registration of the spectral channels can be difficult to achieve in manufacture. Maintaining proper co-registration in use can be even more difficult, as systems with multiple optical paths are particularly prone to deformation due to effects such as thermal expansion and vibration. Accordingly, spectrometers featuring multiple optical paths can be expensive to manufacture and can be difficult to keep calibrated. Alternatively, common-path spectrometers employing dual band detector arrays that can separately detect light of different wavelengths at a single pixel of a detector have been described. However, such detector arrays are difficult and expensive to manufacture. In addition, such detectors have suffered from poor signal to noise characteristics.